SSL/TLS Strong Encryption: How-To

The solution of this problem is trivial
and is left as an exercise for the reader.

-- Standard textbook cookie

How to solve particular security constraints for an SSL-aware
webserver is not always obvious because of the coherences between SSL,
HTTP and Apache's way of processing requests. This chapter gives
instructions on how to solve such typical situations. Treat it as a first
step to find out the final solution, but always try to understand the
stuff before you use it. Nothing is worse than using a security solution
without knowing its restrictions and coherences.

httpd.conf

This facility is called Server Gated Cryptography (SGC) and details
you can find in the README.GlobalID document in the
mod_ssl distribution. In short: The server has a Global ID server
certificate, signed by a special CA certificate from Verisign which
enables strong encryption in export browsers. This works as following:
The browser connects with an export cipher, the server sends its Global
ID certificate, the browser verifies it and subsequently upgrades the
cipher suite before any HTTP communication takes place. The question
now is: How can we allow this upgrade, but enforce strong encryption.
Or in other words: Browser either have to initially connect with
strong encryption or have to upgrade to strong encryption, but are
not allowed to keep the export ciphers. The following does the trick:

Obviously you cannot just use a server-wide SSLCipherSuite which restricts the
ciphers to the strong variants. But mod_ssl allows you to reconfigure
the cipher suite in per-directory context and automatically forces
a renegotiation of the SSL parameters to meet the new configuration.
So, the solution is:

# be liberal in general
SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL

When you know your user community (i.e. a closed user group
situation), as it's the case for instance in an Intranet, you can
use plain certificate authentication. All you have to do is to
create client certificates signed by your own CA certificate
ca.crt and then verify the clients against this
certificate.

httpd.conf

# require a client certificate which has to be directly
# signed by our CA certificate in ca.crt
SSLVerifyClient require
SSLVerifyDepth 1
SSLCACertificateFile conf/ssl.crt/ca.crt

The key is to check for various ingredients of the client certificate.
Usually this means to check the whole or part of the Distinguished
Name (DN) of the Subject. For this two methods exists: The mod_auth based variant and the SSLRequire variant. The first method is
good when the clients are of totally different type, i.e. when their
DNs have no common fields (usually the organisation, etc.). In this
case you've to establish a password database containing all
clients. The second method is better when your clients are all part of
a common hierarchy which is encoded into the DN. Then you can match
them more easily.

Let us assume the Intranet can be distinguished through the IP
network 192.160.1.0/24 and the subarea on the Intranet website has
the URL /subarea. Then configure the following outside
your HTTPS virtual host (so it applies to both HTTPS and HTTP):